Directed energy weapons could have several main advantages over conventional weaponry:

Direct energy weapons can be used discreetly; radiation above and below the visible spectrum is invisible and does not generate sound.[19][20]

Light is only very slightly altered by gravity, giving it an almost perfectly flat trajectory. It is also practically immune (in anything resembling normal planetary conditions) to both windage and Coriolis force. This makes aim much more precise and extends the range to line-of-sight, limited only by beam diffraction and spread (which dilute the power and weaken the effect), and absorption or scattering by intervening atmospheric contents.

Lasers travel at light-speed and have near infinite range and therefore, are suitable for use in space warfare.

Laser weapons potentially eliminate many logistical problems in terms of ammunition supply, as long as there is enough energy to power it the laser ammunition supply is assured

Depending on several operational factors, Directed Energy Weapons may be cheaper to operate than conventional weapons in certain contexts[21]

Although some devices are labelled as microwave weapons, the microwave range is commonly defined as being between 300 MHz and 300 GHz, which is within the RF range[22]—these frequencies having wavelengths of 1–1000 millimeters. Some examples of weapons which have been publicized by the military are as follows:

Active Denial System is a millimeter wave source that heats the water in a human target's skin and thus causes incapacitating pain. It was developed by the U.S. Air Force Research Laboratory and Raytheon for riot-control duty. Though intended to cause severe pain while leaving no lasting damage, concern has been voiced as to whether the system could cause irreversible damage to the eyes. There has yet to be testing for long-term side effects of exposure to the microwave beam. It can also destroy unshielded electronics.[23] The device comes in various sizes, including attached to a Humvee.

Vigilant Eagle is a proposed airport defense system that directs high-frequency microwaves towards any projectile that is fired at an aircraft.[24] The system consists of a missile-detecting and tracking subsystem (MDT), a command and control system, and a scanning array. The MDT is a fixed grid of passive infrared (IR) cameras. The command and control system determines the missile launch point. The scanning array projects microwaves that disrupt the surface-to-air missile's guidance system, deflecting it from the aircraft.[25]

The effective radiated power (ERP) of the EL/M-2080 Green Pine radar makes it a hypothetical candidate for conversion into a directed-energy weapon, by focusing pulses of radar energy on target missiles.[29] The energy spikes are tailored to enter missiles through antennas or sensor apertures where they can fool guidance systems, scramble computer memories or even burn out sensitive electronic components.[29]

AESA radars mounted on fighter aircraft have been slated as directed energy weapons against missiles, however, a senior US Air Force officer noted: "they aren't particularly suited to create weapons effects on missiles because of limited antenna size, power and field of view".[30] Potentially lethal effects are produced only inside 100 metres range, and disruptive effects at distances on the order of one kilometre. Moreover, cheap countermeasures can be applied to existing missiles.[31]

An electrolaser first ionizes its target path, and then sends a powerful electric current down the conducting track of ionized plasma, somewhat like lightning. It functions as a giant, high-energy, long-distance version of the Taser or stun gun.

Pulsed Energy Projectile or PEP systems emit an infrared laser pulse which creates rapidly expanding plasma at the target. The resulting sound, shock and electromagnetic waves stun the target and cause pain and temporary paralysis. The weapon is under development and is intended as a non-lethal weapon in crowd control though it can also be used as a lethal weapon.

A dazzler is a directed-energy weapon intended to temporarily blind or disorient its target with intense directed radiation. Targets can include sensors or human vision. Dazzlers emit infrared or invisible light against various electronic sensors, and visible light against humans, when they are intended to cause no long-term damage to eyes. The emitters are usually lasers, making what is termed a laser dazzler. Most of the contemporary systems are man-portable, and operate in either the red (a laser diode) or green (a diode-pumped solid-state laser, DPSS) areas of the electromagnetic spectrum.

Initially developed for military use, non-military products are becoming available for use in law enforcement and security.[32][33]

In 1991 scientists at the US Army Missile Command developed and field tested a ruggedized tunable laser emitting narrow-linewidth in the yellow-orange-red part of the spectrum.[41] Never went beyond the experimental stage.

On March 18, 2009 Northrop Grumman announced that its engineers in Redondo Beach had successfully built and tested an electric laser capable of producing a 100-kilowatt ray of light, powerful enough to destroy cruise missiles, artillery, rockets and mortar rounds.[42] An electric laser is theoretically capable, according to Brian Strickland, manager for the United States Army's Joint High Power Solid State Laser program, of being mounted in an aircraft, ship, or vehicle because it requires much less space for its supporting equipment than a chemical laser.[43] Experimental.

On April 6, 2011, the U.S. Navy successfully tested a laser gun, manufactured by Northrop Grumman, that was mounted on the former USS Paul F. Foster(DD-964), which was used as a test ship. When engaged during the test that occurred off the coast of Central California in the Pacific Ocean test range, the laser gun was documented as having "a destructive effect on a high-speed cruising target", said Chief of Naval Research Admiral Nevin Carr.[44] While classified, the proposed range of the laser gun is measured in miles, not yards. Experimental.

Most of these projects have been canceled, discontinued, never went beyond the prototype or experimental stage, or are only used in niche applications. Effective, high performance laser weapons seem to be difficult to achieve using current or near-future technology.[18][17][69]

Laser beams begin to cause plasma breakdown in the atmosphere at energy densities of around one megajoule per cubic centimetre. This effect, called "blooming," causes the laser to defocus and disperse energy into the surrounding air. Blooming can be more severe if there is fog, smoke, or dust in the air.

Techniques that may reduce these effects include:

Spreading the beam across a large, curved mirror that focuses the power on the target, to keep energy density en route too low for blooming to happen. This requires a large, very precise, fragile mirror, mounted somewhat like a searchlight, requiring bulky machinery to slew the mirror to aim the laser.

Using a phased array. For typical laser wavelengths, this method would require billions of micrometre-size antennae. There is currently no known way to implement these, though carbon nanotubes have been proposed. Phased arrays could theoretically also perform phase-conjugate amplification (see below). Phased arrays do not require mirrors or lenses, and can be made flat and thus do not require a turret-like system (as in "spread beam") to be aimed, though range will suffer if the target is at extreme angles to the surface of the phased array.[70]

Using a phase-conjugate laser system. This method employs a "finder" or "guide" laser illuminating the target. Any mirror-like ("specular") points on the target reflect light that is sensed by the weapon's primary amplifier. The weapon then amplifies inverted waves, in a positive feedback loop, destroying the target, with shockwaves as the specular regions evaporate. This avoids blooming because the waves from the target pass through the blooming, and therefore show the most conductive optical path; this automatically corrects for the distortions caused by blooming. Experimental systems using this method usually use special chemicals to form a "phase-conjugate mirror". In most systems, the mirror overheats dramatically at weapon-useful power levels.

The Chinese People's Liberation Army has invested in the development of coatings that can deflect beams fired by U.S. military lasers. Laser light can be deflected, reflected, or absorbed by manipulating physical and chemical properties of materials. Artificial coatings can counter certain specific types of lasers, but a different type of laser may match the coating's absorption spectrum enough to transfer damaging amounts of energy. The coatings are made of several different substances, including low-cost metals, rare earths, carbon fiber, silver, and diamonds that have been processed to fine sheens and tailored against specific laser weapons. China is developing anti-laser defenses because protection against them is considered far cheaper than creating competing laser weapons themselves.[71] Apart from creating countermeasures, China has also created a direct-energy weapon called the Silent Hunter that can burn through 5mm of steel at 1000m.[72][73]

Dielectric mirrors, inexpensive ablative coatings, thermal transport delay and obscurants are also being studied as countermeasures.[74] In not a few operational situations, even simple, passive countermeasures like rapid rotation (which spreads the heat and does not allow a fixed targeting point) or higher acceleration (which increases the distance and changes the angle quickly) can defeat or help to defeat non-highly pulsed, high energy laser weapons.[75]

Particle-beam weapons can use charged or neutral particles, and can be either endoatmospheric or exoatmospheric. Particle beams as beam weapons are theoretically possible, but practical weapons have not been demonstrated yet. Certain types of particle beams have the advantage of being self-focusing in the atmosphere.

Blooming is also a problem in particle-beam weapons. Energy that would otherwise be focused on the target spreads out; the beam becomes less effective:

Thermal blooming occurs in both charged and neutral particle beams, and occurs when particles bump into one another under the effects of thermal vibration, or bump into air molecules.

Electrical blooming occurs only in charged particle beams, as ions of like charge repel one another.

The MARAUDER (Magnetically Accelerated Ring to Achieve Ultra-high Directed-Energy and Radiation) used the Shiva Star project (a high energy capacitor bank which provided the means to test weapons and other devices requiring brief and extremely large amounts of energy) to accelerate a toroid of plasma at a significant percentage of the speed of light.[76]

Tests performed on mice show the threshold for both lung and liver damage occurs at about 184 dB. Damage increases rapidly as intensity is increased. Noise-induced neurological disturbances in humans exposed to continuous low frequency tones for durations longer than 15 minutes involved development of immediate and long-term problems affecting brain tissue. The symptoms resembled those of individuals who had suffered minor head injuries. One theory for a causal mechanism is that the prolonged sound exposure resulted in enough mechanical strain to brain tissue to induce an encephalopathy.[78]

According to a legend, Archimedes created a mirror with an adjustable focal length (or more likely, a series of mirrors focused on a common point) to focus sunlight on ships of the Roman fleet as they invaded Syracuse, setting them on fire.[81] Historians point out that the earliest accounts of the battle did not mention a "burning mirror", but merely stated that Archimedes's ingenuity combined with a way to hurl fire were relevant to the victory. Some attempts to replicate this feat have had some success; in particular, an experiment by students at MIT showed that a mirror-based weapon was at least possible, if not necessarily practical.[82]

Stories in the 1930s and World War Two gave rise to the idea of an "engine-stopping ray". They seemed to have arisen from the testing of the television transmitter in Feldberg, Germany. Because electrical noise from car engines would interfere with field strength measurements, sentries would stop all traffic in the vicinity for the twenty minutes or so needed for a test. Reversing the order of events in retelling the story created a "tale" where tourists car engine stopped first and then were approached by a German soldier who told them that they had to wait. The soldier returned a short time later to say that the engine would now work and the tourists drove off. Such stories were circulating in Britain around 1938 and during the war British Intelligence relaunched the myth as a "British engine-stopping ray", trying to spoof the Germans into researching what the British had supposedly invented in an attempt to tie up German scientific resources.[83]

Among the directed-energy weapons the Nazis investigated were X-ray beam weapons developed under Heinz Schmellenmeier, Richard Gans and Fritz Houtermans. They built an electron accelerator called Rheotron (invented by Max Steenbeck at Siemens-Schuckert in the 1930s, these were later called Betatrons by the Americans) to generate hard X-ray synchrotron beams for the Reichsluftfahrtministerium (RLM). The intent was to pre-ionize ignition in aircraft engines and hence serve as anti-aircraft DEW and bring planes down into the reach of the flak. The Rheotron was captured by the Americans in Burggrub on April 14, 1945.

Another approach was Ernst Schiebolds 'Röntgenkanone' developed from 1943 in Großostheim near Aschaffenburg. The Company Richert Seifert & Co from Hamburg delivered parts.[85]

Though the strategic missile defense concept has continued to the present under the Missile Defense Agency, most of the directed-energy weapon concepts were shelved. However, Boeing has been somewhat successful with the Boeing YAL-1 and Boeing NC-135, the first of which destroyed two missiles in February 2010. Funding has been cut to both of the programs.

During the Iraq War, electromagnetic weapons, including high power microwaves, were used by the U.S. military to disrupt and destroy Iraqi electronic systems and may have been used for crowd control. Types and magnitudes of exposure to electromagnetic fields are unknown.[89]

The Soviet Union invested some effort in the development of ruby and carbon dioxide lasers as anti-ballistic missile systems, and later as a tracking and anti-satellite system. There are reports that the Terra-3 complex at Sary Shagan was used on several occasions to temporarily "blind" US spy satellites in the IR range.

It has been claimed (and proven false) that the USSR made use of the lasers at the Terra-3 site to target the Space Shuttle Challenger in 1984.[90][91] At the time, the Soviet Union were concerned that the shuttle was being used as a reconnaissance platform. On 10 October 1984 (STS-41-G), the Terra-3 tracking laser was allegedly aimed at Challenger as it passed over the facility. Early reports claimed that this was responsible for causing "malfunctions on the space shuttle and distress to the crew", and that the United States filed a diplomatic protest about the incident.[90][91] However, this story is comprehensively denied by the crew members of STS-41-G and knowledgeable members of the US intelligence community.[92]

The TECOM Technology Symposium in 1997 concluded on non-lethal weapons, "determining the target effects on personnel is the greatest challenge to the testing community", primarily because "the potential of injury and death severely limits human tests".[94]

Also, "directed energy weapons that target the central nervous system and cause neurophysiological disorders may violate the Certain Conventional Weapons Convention of 1980. Weapons that go beyond non-lethal intentions and cause "superfluous injury or unnecessary suffering" may also violate the Protocol I to the Geneva Conventions of 1977."[95]

Some common bio-effects of non-lethal electromagnetic weapons include:

^Richard D. Fisher, Jr. (February 23, 2017). "China's Progress with Directed Energy Weapons"(PDF). p. 8. A Poly video showed this laser could 'ablate' or penetrate five 2 millimeter steel plates at a range of 800 meters, and an official stated it could penetrate 5 millimeters of steel at 1,000 meters.